pcmenc: set frame_size to 0.

This indicates that the actual frame size is based on the buf_size passed to
avcodec_encode_audio().

libavcodec/pcm.c

@@ -33,7 +33,7 @@
 
 static av_cold int pcm_encode_init(AVCodecContext *avctx)
 {
-    avctx->frame_size = 1;
+    avctx->frame_size = 0;
     switch(avctx->codec->id) {
     case CODEC_ID_PCM_ALAW:
         pcm_alaw_tableinit();